free height of center plate wearing surface from rail top

例句：

1. 1.The free height of the building is 20 meters.
   这座建筑的自由高度为20米。
2. 2. The elevator has a free height of 15 meters.
   电梯的自由高度为15米。
3. 3.The tallest building in the city has a free height of 60 meters.
   该市最高的建筑物自由高度为60米。